1. Field of the Invention
The present invention relates to a control system in a vehicle provided with a continuous variable transmission, wherein the throttle valve can be controlled regardless of the operation of an accelerator pedal.
2. Description of the Related Art
As is well known to those skilled in the art, the horsepower obtained from an internal combustion engine is determined by engine speed and torque, and there are combinations of engine speed and torque values which enable the fuel consumption to be minimized. Therefore, a system for driving a vehicle through a continuous variable transmission has been proposed in which the engine output is continuously varied regardless of the operation of the accelerator pedal. In this system, the target values of the engine speed and torque are determined in accordance with the value of the horsepower, to attain a minimum fuel consumption. The speed ratio of the transmission is controlled so that the speed of the input shaft of the transmission conforms to the target engine speed, and the degree of opening of the throttle valve is controlled so that the engine torque is equal to the target torque. In such a system, the target torque of the engine is obtained by calculating the target horsepower or a target driving torque of the vehicle. (See, for example, Japanese Unexamined Patent Publication (Kokai) No. 58-160661.) This system is advantageous in that both a low fuel consumption and a highly efficient acceleration operation, which are, per se, opposing requirements, are attained, since a required engine output power or driving torque of the transmission is obtained even when the engine is in a transient state such as an acceleration condition.
In the known system, to obtain a target engine torque, a target opening of the throttle valve is calculated from the target engine horsepower or target transmission driving torque, and a feedback control is effected so that the throttle opening is equal to the target value. However, this throttle opening control makes it difficult to obtain a correct target torque value, because of the non-linearity between the throttle opening and the engine torque. When the engine is under a low load, a large change in the intake air amount is generated by a small change in the degree of opening of the throttle value, causing a large change in the engine torque. Furthermore, even when a same degree of throttle opening is maintained, the amount of intake air actually introduced is changed by a change in the atmospheric pressure, causing a change in the engine torque.